The use of spinal fixation instrumentation to align and/or fix a desired relationship between adjacent vertebral bodies is well established. Such instrumentation typically includes a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to screws which have been inserted into the patient's vertebrae or to spinal hooks which can be placed into a vertebral arch for coupling to the vertebral bodies. Once installed, the spinal fixation instrumentation holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
One example of a rod based spinal fixation system is provided in U.S. Pat. No. 5,005,562, issued Apr. 9, 1991 to Cotrel (which is hereby incorporated by reference). This system includes pedicle screw and spinal hook vertebral coupling elements having integral U-shaped bodies that extend outward from the vertebrae to which they are attached. A spinal fixation rod is shaped as desired and fitted into the “U” of U-shaped bodies of adjacent vertebrae. The inner surfaces of the U-shaped body are threaded to accept a set screw, and rod is fixed to the vertebral coupling elements by threading a set screw into each of the U-shaped bodies to lock in the rod.
U.S. Pat. No. 5,545,165, issued Aug. 13, 1996 to Biedermann et al. (and incorporated herein by reference), illustrates an improvement in closure systems for fixing a rod to vertebral coupling elements over those provided by Cotrel. The Biedermann et al. system also uses pedicle screws and spinal hooks having U-shaped bodies that extend outward from the vertebrae to which they are attached. The U-shaped bodies of the Biedermann et al. system are threaded on both the inside and the outside. The rod is therefore locked in by both an inner set screw and an outer lock nut. In the illustrated embodiments, the inner set screw is adapted to be driven on its threads using a hex-shaped driver element, and the outer locking nut is provided with hex-shaped flat outer surfaces suitable for engagement with a wrench or similar driving tool.
When using screws and rods in spinal surgery, compression between the screws is often desired, or even required. For some procedures, compression of the spine between screws can help to conform the spine to a desired profile (such as when correcting a scoliotic spine). For many procedures, compression is desired to place a load on bone graft material that has been loaded into the disc space in order to promote healing and rapid fusion. In response to this need, conventional compressor tools have been developed. These conventional compressor tools can be generally shaped like a common pair of pliers, specially adapted to contact screws placed in adjacent vertebrae so that a surgeon can squeeze the handle portion of the conventional compressor tool to compress a region of the spine being treated.
Many surgeons are now treating spinal pathologies (spondylolysis, degenerative disc disease, scoliosis, etc.) through smaller and smaller incisions or portals (an approach commonly known as minimally invasive surgery). For example, many surgeons now prefer to treat anterior spinal deformities thoroscopically. In order to accomplish surgery in this manner, each of the instruments used in the surgery must be designed to fit into either the incision or portal and be articulated with minimal trauma to the surgical site. Conventional compressor tools include pivot regions that grow in dimension about the pivot when the tool is articulated. This makes conventional compressor tools unusable in minimally invasive surgery.
In an attempt to overcome this problem, one style of compressor has been designed to have a rack and pinion system. This system is inserted into the surgical site so that the pinion may be articulated with a tool such as a screwdriver. Still another style of compressor uses a cable system that is wrapped around the implanted screws. By increasing tension on the cable, compression is created. Each of these designs has its own set of problems however, including the amount of time it takes to set up and use the tools, and the extra mechanical advantage that the tools provide. Extra mechanical advantage can be a problem in that surgeons must learn how much force is applied by the device, as opposed to force provided by the surgeon, so that the surgeon can be careful not to apply too much compression.
Accordingly, a need exists for a compressor tool for surgeons who choose to perform spinal fixation surgery using minimally invasive techniques and without the problems of known compressor tools.